This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We rely on our innate immune system as the first line of defense response against invading pathogens. This immune response is critically dependent on the Toll-like receptors (TLRs). Each TLR recognizes a different molecular pattern that is characteristic of specific pathogens. Upon ligand binding, TLRs transmit a signal to the nucleus that leads to the production of proinflammatory compounds. We will determine three-dimensional structures of a TLR in complex with its ligand. This structure will provide critical insight into the molecular basis of pathogen recognition and proinflammatory signal generation. To test our structure-based hypotheses on TLR function, we will measure the effect on immune signaling of engineered mutations that are predicted from the structures to interfere with ligand binding or signal generation. We propose a strategy to seek high-affinity TLR ligands, or agonists. Our work will reveal the molecular basis for how pathogen recognition is translated into an immune response signal. Our structure will guide efforts to design synthetic TLR agonists, which could serve as novel vaccine adjuvants, or as immunomodulatory therapeutics. Such therapeutics would provide a powerful new means to prevent and treat infectious diseases.